Subterranean drilling is an expensive process during which a bore hole is drilled through the earth to gain access to a desired resource such as an oil or gas deposit. Many drilling operations employ directional drilling, especially when the target deposit is located laterally thousands of feet from the drilling rig. The length of the bore hole required to reach the deposit is determined by the depth and lateral displacement of the deposit from the drilling rig.
The drilling process typically involves rotating a drill bit with a downhole motor at a remote end of a string of drill pipe or “drill string.” The rotating bit bores through underground formations, opening a path for the drill string. Drilling fluid forced through the drill string powers the downhole motor. Directional drilling is used to steer the motor and bit from a straight drill path in any inclination and azimuthal orientation, and allows an operator to guide the bore hole to the target deposit. For example, to access an underground deposit, the operator can drill a vertical bore hole from the drilling rig. The operator then steers the downhole motor and drill bit to drill a deflected continuation of the bore hole to reach and penetrate the deposit. In some instances, the bore hole can have one or more substantially horizontal sections including where the bore hole penetrates the deposit.
Significant friction can exist between the bore hole and the drill string. Friction generally slows the drilling process by reducing the force applied to the drill bit. Friction is most significant where the drill string is forced against the bore hole such as in regions where the bore hole is substantially horizontal. During straight path drilling, the drill string is continuously rotated in a single direction about its longitudinal axis to reduce the effect of friction and increase the penetration rate.
Directional drilling is typically accomplished by orienting the toolface of the drilling bit in the desired direction and maintaining the orientation. To start directional drilling, the continuous rotation of the drill string is terminated and the operator determines the current toolface orientation, for example, by measuring the toolface orientation using “measurement while drilling” (MWD) sensors. The drill string is then rotated to change the direction of the toolface to a desired direction for subsequent drilling of the bore hole.
Directional drilling is often performed at the end of a drill string that is several thousand feet in length. Although change of the bore hole direction is typically accomplished through a gradual deflected over hundreds of feet or more so that the drill string bends gradually, the friction between the drill string and the bore hole generally increases. In addition, the drill string is elastic and stores torsional tension like a spring. Consequently, when an operator makes a static angle adjustment to the drill string at the drilling rig to change the toolface orientation, a substantial portion of the angle adjustment is “absorbed” by the friction without changing the toolface orientation. Thus the drill string can require more rotation at the surface than the desired rotation of the toolface.
Similar to straight path drilling, the rate of penetration during directional drilling is adversely affected by friction between the drill string and bore hole. To reduce frictional limitation of the penetration rate, the drill string can alternate between rotation in forward (e.g., clockwise) and reverse (e.g., counterclockwise) directions. Due to the torsional spring properties of the drill string, the rotation of the drill string at the surface does not match the rotation of the drill string at other positions along its length. More specifically, the rotation of the drill string decreases with distance from the drilling rig. If the amount of rotation imparted at the surface is properly limited, the drill string will not rotate at the downhole motor. Thus the frictional limitation on the drilling process can be reduced by back and forth rotation of the surface portion of the drill string within appropriate angular limits without changing the orientation of the toolface although in practice it can be difficult to achieve the desired back and forth rotation without affecting the orientation of the toolface during directional drilling.
Thus, there remains a need for a method of directional drilling that overcomes the above described problems. The method of the current invention satisfies this need and provides additional advantages.